1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus in which an improvement is made in a toner supplying method.
2. Description of the Prior Art
A copying machine, a laser printer or the like generally includes an image forming portion in which an electrostatic latent image formed on a photoconductor is caused to be visible by means of toner in two-constituent developer and the toner image is transferred onto a sheet of copy paper or the like. In such an image forming apparatus, the toner contained in the developer stored in a developer container is consumed and decreased as a result of development of an electrostatic latent image and consequently it is necessary to newly supply a suitable amount of toner.
For that reason, various methods such as indicated below are conventionally utilized.
(1) Method in which a toner density in developer is detected by making use of the phenomenon that inductance of a coil of a magnetic sensor provided in a developing device changes due to change in the toner density in the developer, and an additional amount of toner is supplied to the developer based on the detected value during operation of the developing device.
(2) Method in which an electrostatic latent image for control of toner supply is formed and developed on a surface of a photoconductor by using, for example, a reference chart having a prescribed image density, prior to copy operation, to optically detect a density of the developed image and an additional amount of toner is supplied to the developer based on the detected value during operation of the developing device.
However, the amount of toner supplied by those methods is set based on an amount of toner to be consumed by image forming operation using a document having an average density of image. In addition, toner supply by those methods is made in a limited short period, for example, during image forming operation.
Accordingly, for example if toner is excessively consumed because of successive copying of documents having high image densities and if toner supply continues with a toner density in developer being lower than a prescribed value, it sometimes happens that the toner density in the developer in the developing device does not attain again the prescribed value since the toner supply is stopped in a short period of time. As a result, copy operation continues with a decreased image density.